This invention relates generally to electrical coupling devices and more particularly, it relates to Partial Discharge Analysis (PDA) coupling devices containing a flexible joint.
A Partial Discharge Analyzer is an electronic instrument that is used for on-line partial discharge measurements on rotating machines, such as motors and generator stator systems. For example, the Partial Discharge Analyzer may be used to monitor the condition of generator stator winding insulation. A Partial Discharge Analyzer uses one or more PDA coupling devices in order to carry out the measurements.
The PDA coupling devices (i.e., PDA couplers) may be installed connected directly on the electrical connection to the machine being monitored. For example, the high voltage side of a PDA coupling device may be connected to the generator stator winding leads and the opposite end of the PDA coupling device may be electrically connected either directly to ground or through an impedance to ground. The PD signals are then detected by various means as they go through the coupler to ground.
One type of prior art PDA coupling device is shown in FIG. 1. The prior art device 1 comprises a cylindrical shell 2 containing a cavity 4. The cavity is filled with an electrical grade potting material 6. The potting material 6 encapsulates a first capacitor 8 and a second capacitor 10. The capacitors 8 and 10 are connected to each other by a first rigid metal rod 11. The first rod 11 screws into the threaded openings in the capacitors 8 and 10. A second rod 12 exits the top of the shell 2 through opening 14 for electrical contact with an external electrical lead, such as a wire (not shown). The shell 2 also contains a second opening 16 in the bottom surface through which a third rod 13 exits.
In general, electrical grade potting and shell materials have larger coefficients of thermal expansion than metals or ceramics. Hence, during thermal cycling of the coupling device, large tensile loads are placed on the ceramic capacitors due to the difference in coefficients of thermal expansion between the rigid metal rod 11 and/or the capacitors 8, 10 and the potting material 6 and/or the shell 2. In other words, the potting material 6 and/or the shell 2 expand at a greater rate than the rigid metal rod 11 and/or the capacitors 8, 10 in response to an increase in temperature. The potting material and/or the shell thus exerts a tensile stress (in the up/down direction in FIG. 1) on the rod 11 and on the ceramic capacitors 8, 10. This is particularly true when one or more terminal leads 12, 13 of a PDA coupling device are fixed by a bolt or other hardware.
Ceramic capacitors do not withstand tensile loads well, and consequently often suffer failure (such as breakage or cracking) upon application of heat to the shell 2 during the operation of the machine being monitored. The PDA coupling device is rendered non-functional upon the failure of the capacitors 8, 10. The present invention is directed to overcoming or at least reducing the problem set forth above.
In accordance with one aspect of the present invention, there is provided a device, comprising a first capacitor, a second capacitor and a flexible connector electrically connecting the first capacitor and the second capacitor.
In accordance with another aspect of the present invention, there is provided a partial discharge analysis coupling device, comprising a shell and a potting material inside the shell encapsulating a first capacitor and a flexible connector electrically connected to the first capacitor.
In accordance with another aspect of the present invention, there is provided a partial discharge analysis coupling device, comprising a shell and a potting material inside the shell encapsulating a first capacitor, a second capacitor and a first means for electrically connecting the first capacitor and the second capacitor and for relieving a tensile load on the first capacitor and the second capacitor during thermal expansion of at least one of the shell and the potting material.
In accordance with another aspect of the present invention, there is provided a device, comprising a shell and a potting material inside the shell encapsulating at least one of a first electrical component, a second electrical component and a flexible connector electrically connecting the first electrical component to the second electrical component.
In accordance with another aspect of the present invention, there is provided a method of making a device, comprising connecting a first electrical component to a second electrical component by a flexible connector and encapsulating at least one of the first electrical component, the second electrical component and the flexible connector with a potting material.